1. Field of the Invention
This invention is directed to a distance measuring apparatus and methods, and, in particular, to a fiber optic based laser ranger finder.
2. Background of the Invention
Large-scale metrology includes the measurement of coordinates over large distances, for example, greater than the volume of a conventional coordinate measurement machine (CMM), which is typically limited to a cube of a few meters. There are many instances, for example in the aircraft industry, radio telescope and linear accelerator applications, where measuring such distances is preferably accomplished with a high degree of accuracy. There are also field measurements of smaller scale which do not lend themselves to placement in a CMM, such as in situ measurements of machinery, outdoor measurements, shop floor measurements, and other well known approaches.
The measurement of coordinates is typically accomplished by measuring a distance and two angles, as with a conventional surveying total station or laser tracking interferometer; the measurement of three orthogonal distances, as with a CMM; the measurement of angles from two locations on a known baseline, such as with theodolites; the measurement of spacing on a two-dimensional image projection from multiple locations, as with photogrammetry; the measurement of distance from three, or more, known locations on a baseline, as with multilateration; and various other manners. “Large-Scale Metrology—An Update”, Estler, et. al., CRP Annals—Manufacturing Technology 51(2), 2002, discusses various techniques for large scale metrology.
Laser rangefinders measure distance by measuring the time taken to propagate an optical wavefront from the emitter to target and back and inferring distance from a known or predicted propagation speed. One modality of time measurement is to modulate the amplitude of the CW emitted wavefront and measure the phase difference of the received wavefront modulation with respect to the transmitted one. Techniques of this type are described in Electronic Distance Measurement, J. M. Rueger, 3rd. Ed., Springer-Verlag, New York, 1990. “Sources of error in a laser rangefinder”, K. S. Hashemi, et al., Rev. Sci. Instrum. 65(10) October 1994 discusses a laser rangefinder and the associated sources of error.
The National Radio Astronomy Observatory (NRAO) Robert C. Byrd Green Bank Telescope (GBT) is a 100 m diameter advanced single dish radio telescope designed for a wide range of astronomical projects with special emphasis on precision imaging. Open-loop adjustments of the active surface, and real-time corrections to pointing and focus on the basis of structural temperatures already allow observations at frequencies up to 50 GHz. Operation at higher frequencies requires more precise knowledge of optical element position and pose.
Limitations in the prior art do not permit the requisite length measurement accuracy. Limitations include variable phase delays in the detector due to beam spot position uncertainty, run-out of the steering mirror leading to systematic range errors, low rates of zero-point measurements (and equivalently, low chopping rates) leading to inclusion of low-offset frequency phase noise errors, and imprecise measurement of the coupling from transmitter to receiver electronics and optics leading to cyclic errors. The ambiguity in absolute range in single modulation CW rangefinders is also problematic, necessitating prior knowledge of range to less than one-half of the ambiguity range. In addition, economies can be obtained if the control and electro-optics needed for measurement can be multiplexed amongst diverse baselines.